The present invention relates to devices constructed from multiple layers of material which are bonded together, and which devices are subject to bursting forces from an internal pressurized fluid. More particularly, the invention finds application in the construction of pressure sensors, particularly those fabricated with brittle materials using micromachining techniques.
As used herein, "micromachined" and derivative words refer to an article which can be fabricated at least in pan using solid state processing techniques such as photolithography, chemical and plasma etching, chemical vapor deposition, plasma deposition, and similar processes known to those skilled in the art.
In FIG. 1, a pressure P to be measured acts on a prior art pressure transducer 10 which includes an upper silicon part 12 and a substrate 14. Part 12 is shaped to define a diaphragm 16 therein and an inner cavity 18. Formed in or near diaphragm 16 are piezoresistive gauge resistors 22,24, at least one of which has a resistance value that varies with the amount of strain in diaphragm 16, which strain is a function of pressure P. A measurement circuit, not shown, couples to resistors 22,24 and provides an output as a function of pressure P.
A difficulty with prior art devices such as that of FIG. 1 occurs if they are subjected to high tensile stresses from pressurized fluids within the device. For example, if transducer 10 is used in a differential pressure application, cavity 18 can be charged with a highly pressurized fluid through orifice 20. If an internal pressure P' greatly exceeds outer pressure P, the stress magnitude 26 along an interface 28 between part 12 and substrate 14 increases rapidly toward a terminus 30 of interface 28 (see FIG. 2), due to the shape of layers 12,14 in the vicinity of interface 28. If a preexisting microscopic crack or other flaw 29 is present at terminus 30, a stress intensity factor associated with such crack or flaw 29 will be relatively high.
In the art of mechanical engineering, the "stress" at any given point in a structure is the net force acting on an arbitrarily small area at the point divided by such area. Stress is a vector quantity, measured in units of Pascals (1 Pa=1 N/m.sup.2). In contrast, "stress intensity factor" is a scalar quantity used in the analysis of an existing crack front in a mechanical structure, having units of Pa.cndot..sqroot.m (or, equivalently, N/m.sup.3/2). If the stress intensity factor exceeds a critical value, referred to as the "fracture toughness" of a material, the crack front will grow, resulting in a fracture in the material. The fracture toughness for silicon, for example, is conservatively estimated at about 7.times.10.sup.5 Pa.cndot..sqroot.m. The reader is referred to Elementary Engineering Fracture Mechanics by David Broek, published 1986, for further background discussion of stress intensity factor and fracture toughness.